Wireless communication systems may be deployed in a number of ways. For example, cellular type systems are typically deployed using an omni deployment or a sectored deployment. In FIG. 1, an omni deployment 10 is shown. Omni deployments are made up of sites with a base station in each site wherein the base stations use transmission (reception) antenna(s) that radiate (receive) nearly uniformly in all directions in the horizontal plane. Omni deployments are typically used for rural, micro-cellular, and indoor areas.
To increase system capacity in densely populated areas, for example, a sectored deployment may be used. An example of a sectored deployment 50 is shown in FIG. 2. Sectored deployments are also made up of sites (i.e. site 1 and site 2) wherein each site includes a base station. In sectored deployments, however, each site is broken down into a plurality of sectors. To provide coverage to each sector, the base stations include a transceiver and antenna(s) for each of their respective sectors so that the antenna(s) in each sector radiate/receive within a different arc. For example, base station 52 has transceivers 56a, 58a, 60a which each radiate with an arc of 120 degrees for sectors 56b, 58b, 60b. For simplicity, the area covered by a transceiver is hereafter referred to as a cell with the understanding that a cell may correspond to either a sector in a sectorized deployment or an area covered by a site in an omnidirectional deployment.
Referring now to FIG. 3, and continuing with cellular type systems purely by way of example, cellular systems generally divide the time axis into continuing intervals of equal duration called frames. A frame 100 is divided into a finite number (Nt) of intervals of equal duration called timeslots. A particular cell is allowed to use some or all of the timeslots for uplink or downlink transmissions as defined by the cell's timeslot assignment. Within each timeslot, it is possible to assign a finite number of codes (Nc) to different WTRU(s) for transmission/reception of wireless signals using spread spectrum technology. The combination of a code and a timeslot is considered a channel and the allocation of the codes to a set of WTRU(s) is referred to as channel allocation. An optimum allocation will reduce interference and increase capacity of the system.
Typically, when a WTRU requests resources (i.e. a set of channels) from a wireless system the WTRU first chooses the base station from which the WTRU has measured the least amount of path loss. Generally, to measure path loss the WTRU subtracts the measured power at which the beacon signals are received from the known transmission power of those signals. The transmission power of the beacon signals is normally signaled within the beacon as part of the system information. The base station from which the least amount of path loss is measured is referred to as the base station that is closest to the WTRU. Note, however, that this base station may not always be the closest in the geographical sense. Once the closest base station has been selected, resources are then allocated to the WTRU. Typically, the channel that is allocated is within a timeslot where the WTRU will experience the least amount of interference.
Interference may originate from two sources, intra-cell and inter-cell. Intra-cell interference is interference seen by a WTRU due to other WTRUs in the same cell as the WTRU. Inter-cell interference is interference seen by the WTRU from WTRUs in other cells. Some wireless communication systems employ some type of multi-user detection (MUD) that cancels most of the intra-cell interference seen by the WTRU. The TDD modes (3.84 Mcps and 1.28 Mcps) of UTRA are examples of such systems. In these systems, channel allocation is primarily concerned with minimizing the inter-cell interference seen by the WTRU.
With respect to inter-cell interference, neighboring cells are typically timeslot synchronized so that the neighboring cells are using the same timeslots for uplink and downlink transmissions. Assigning resources to a WTRU in one cell may result in a significant increase in inter-cell interference to the neighboring cells. For example, the WTRU may be assigned to a timeslot that, although being the timeslot with the least amount of interference, the addition of the WTRU causes a sudden increase in interference that cannot be compensated by WTRUs operating in the same timeslot in neighboring cells (i.e. WTRUs assigned resources from that timeslot cannot transmit with enough power to maintain a satisfactory signal to interference plus noise ratio (SINR)). The above outage can occur due to the well-known power balancing effect between the WTRU and WTRUs from neighboring cells that are also assigned resources in the same timeslot.
Power balancing is a phenomenon that occurs in various wireless communication systems such as CDMA type systems, for example. In CDMA type systems, since all WTRUs within the system share the frequency spectrum, each WTRU sees the other WTRUs noise/interference. To achieve reliable communication, a SINR ratio must be above a certain ratio. When a new WTRU is added to a system, the interference in the system increases. This causes existing WTRUs within the system to increase their power to maintain the SINR above the certain ratio. The power increase from the existing WTRUs causes an increase in interference to the new WTRU. The new WTRU then increases its power again to maintain the SINR. This pattern continues until the SINR stabilizes. In cases where the SINR does not stabilize, one or more WTRU's will experience outage. In the prior art, there is a mechanism to predict the increase in interference, based on the current level of interference and relative path loss of the new WTRU to the system. This mechanism can be used to choose the timeslot with the lowest predicted increase in interference. However, there may be cases where, although resources within the timeslot in this cell and neighboring cells are not fully used (i.e. the MUD benefits are not fully realized), there is no suitable timeslot within this cell for the new WTRU in this cell. This will lead to a blocked WTRU admission.
It would therefore be desirable to have a method and system for assigning resources in wireless communication systems without such limitations.